The field of the invention is that of telecommunications, to be more precise that of cellular telecommunications networks.
This document distinguishes between two types of cellular network, namely:                cellular networks operating in a frequency band allocated to a telecommunications operator with the use thereof being subject to the operator being authorized; these include second generation cellular GSM Edge Radio Access Networks (GERAN), third generation UMTS Terrestrial Radio Access Networks (UTRAN), and other networks (AMPS, D-AMPS, etc.); and        cellular networks formed by local area telecommunications networks connected to a cellular network of the first type, in particular home local area networks managed by a Generic Access Network (GAN) architecture as defined by 3GPP Release 6; these local area networks are not limited to wireless local area telecommunications networks operating in public frequency bands not allocated to operators, and they also include cable local area networks, for example Ethernet networks; throughout the remainder of this document these networks are referred to as “local area networks”.        
The architecture of public cellular networks is one in which base stations, called base transceiver stations (BTS) in a GERAN and Nodes B in a UTRAN, are controlled by controllers, called base station controllers (BSC) in a GERAN and radio network controllers (RNC) in a UTRAN, that are connected to a “circuit” core network and to a “packet” core network.
In cellular GAN, cells are defined by access points (AP) enabling users to access mobile services via an Internet Protocol (IP) connection set up with an element of the network called the GAN controller (GANC), responsible for controlling access points. Thus an access point constitutes a physical entity enabling a mobile user to be connected to the GANC via an IP connection.
In accordance with the above-mentioned standard, the GANC of the GAN are connected to the core network of the cellular GERAN and are seen by that core network as a BSC of the cellular GERAN architecture.
GANs extend the coverage of second generation networks by deploying these access points. Thus users, especially domestic users, can continue to access services of public cellular GERAN via their local area network access points rather than via the base station of the public GERAN.
The invention is more particularly concerned with the handover mechanism, i.e. the mechanism for connection transfer during a call, managed by the network and guaranteeing continuity of service to a terminal moving from one cell to an adjoining cell.
In the prior art, the BSCs or the RNCs manage handover of a mobile between two GERAN cells or between a GERAN cell and a UTRAN cell. The transfer can be initiated on the basis of signal levels of the attachment cell to which the mobile is attached and of the adjoining cells as measured by the mobile terminal and sent back to the network.
For example, in a GERAN, the mobile terminal sends back the signal levels of its attachment cell and of the six best adjoining GERAN cells. On receiving these measurements, the BSC can decide to transfer the mobile to another cell, which will then become its attachment cell. The handover algorithm is based on comparing the signal level of the current attachment cell to a threshold configured in the network. If the signal level is below the threshold, the future attachment cell is that with the highest signal level above the threshold provided that its level is above the threshold.
When a mobile is in the coverage areas of both a GERAN and a UTRAN, and is attached to the access GERAN, the BSC sends the identities of the adjoining GERAN and UTRAN cells to the mobile terminal in an information message. The mobile terminal periodically sends back the signal levels of the best six adjoining cells.
When a mobile terminal is in the coverage areas of both a GERAN and a UTRAN and is attached to the access UTRAN, it sends back the signal levels of the adjoining cells either periodically or when an event occurs, the RNC sending the type of initiator event to the mobile terminal.
The standard defines a number of events, including four events for measurements effected on the various types of access network:                Event 3A: the estimated quality of the current UTRAN frequency is below a certain threshold and the estimated quality on the other system (e.g. the GERAN system) is above a certain threshold.        Event 3B: the estimated quality on the other system (e.g. the GERAN system) is below a certain threshold.        Event 3C: the estimated quality on the other system (e.g. the GERAN system) is above a certain threshold.        Event 3D: change of the best cell in the other system (e.g. the GERAN system).        
According to the standard, a GAN cell is configured in the BSCs and the RNCs of the public cellular networks as a GERAN adjoining cell. Consequently, the handover mechanism between a GAN cell and a GERAN cell, or between a GAN cell and a UTRAN cell, proceeds in substantially the same manner as handover within a GERAN, or as handover between a GERAN and a UTRAN.
The 3GPP standard TS 43.318: “Generic access to the A/Gb interface”, Stage 2, v6.3.0 provides for users to specify their preferred attachment network. This preference, or mode, can be chosen from:                “GAN preferred” mode: the user wishes to access mobile services via the GAN as soon as it is available. If it is not available, the user uses the available access network (GERAN or UTRAN).        “GERAN/UTRAN preferred” mode: the user wishes to access mobile services via the access GERAN or UTRAN as soon as these are available. If they are not available, the user uses the GAN.        “GERAN/UTRAN only” mode: the user wishes to access mobile services only via the access GERAN or UTRAN.        “GAN only” mode: the user wishes to access mobile services only via the GAN.        
Unfortunately, the 3GPP specification 43.318 cannot implement the “GERAN/UTRAN preferred” mode because it specifies that, when a mobile is in the dual coverage of a GAN and a public cellular network, it should send back to the network a maximum signal level for the GAN cell (i.e. a dummy signal level with the value 63), so that the GAN cell is always given preference over the cells of the public cellular network and is therefore chosen automatically by the network as the target cell for handover.
Similarly, according to the standard, if a mobile terminal is in the double coverage area of a public cellular network and a GAN, and if its attachment network is the GAN, then handover does not take account of the user's preference. Transfer can be initiated either by the network or by the mobile terminal.
When handover is initiated by the network, the network sends a GA-CSR UPLINK QUALITY INFORMATION message to the mobile terminal. The 44.318 standard specifies that when the quality stated in a GA-CSR UPLINK QUALITY INFORMATION message indicates a “radio problem” or an “undetermined problem”, the mobile terminal must select another AP (i.e. it must remain in the intra-GAN mode), while for other types of quality the mobile terminal must initiate handover to the GERAN/UTRAN by sending a GA-CSR HANDOVER INFORMATION message that contains a list of the adjoining GERAN and UTRAN cells and their respective signal levels. When handover is initiated by the mobile terminal, the terminal sends a GA-CSR HANDOVER INFORMATION message directly to the network.
This mechanism does not implement the “GERAN/UTRAN preferred” mode because initiation of handover is independent of the user's preference.